1. Field of the Disclosure
The disclosure relates to a gas sensor comprising a housing which is adapted to be evacuated and is closed by a gas-selectively permeable membrane, and comprising a heating device arranged on the membrane.
2. Discussion of the Background Art
DE 100 31 882 A1 (Leybold) describes a gas-selective sensor for helium or hydrogen, which sensor is provided with a housing made of glass or another silicon material and is closed by a selectively gas-permeable membrane. Accommodated in the housing is a gas-pressure sensor for measuring the gas pressure. After evacuation, the resultant gas pressure occurring in the interior of the housing will be indicative of the quantity of the test gas passing through the membrane. The membrane is a silicon disk provided with window-like perforations. In each of said perforations, a helical heating element made of platinum is arranged on a membrane wall. The helical heating elements in their totality form a heating arrangement for heating the membrane so that the membrane will develop the required properties of selective gas-permeability.
EP 0 831 964 B1 (Leybold) describes the production process for a selectively gas-permeable membrane made of silicon. In said process, a silicon disk will on both of its sides be provided with oxidation layers and then be etched, thus maintaining a continuous SiO2 layer only on one side of the disk. In between, window-like perforations are formed for arranging helical heating elements therein by use of methods from the field of thin-film technology.
Described in DE 10 2004 034 381 A (Inficon) is a selective gas sensor wherein the housing is likewise closed by a membrane and includes a gas pressure sensor.
The above mentioned gas sensors are also referred to as quartz window sensors. Gas sensors with gas-selective membranes are used for leak detection when a container or conduit has to be tested for leaks. The test object will be filled with a test gas such as e.g. helium. Then, with the aid of a test-gas sensor, possible escape of test gas is checked externally of the test object. Often, for this purpose, the object filled with test gas is placed in a test chamber which together with an associated membrane sensor will be evacuated, thus allowing test gas to issue through possible leaks and to be detected by the membrane sensor. A quartz window sensor with heated membrane must have a very stable membrane temperature to prevent the test-gas signal from drifting. A drift of this signal will immediately enter the limit of detection. In case of a membrane with temperature-regulated heating device, the pump-off process during the evacuating of the test chamber is accompanied, due to the rapid pressure drop in the test chamber, by a distinct change of heat dissipation from the housing walls of the sensor, thus causing a signal drift to occur in spite of the temperature regulation. Consequently, only relatively large test-gas signals can be measured or, otherwise, one would have to wait for the lapse of unacceptably long settling times.
It is an object of the disclosure to provide a gas sensor wherein the desired temperature of the membrane is maintained with high constancy so that the stability of the signal of the gas sensor is increased and also small leakage rates can be reliably measured.